1. Field of the Invention
The present invention relates to a method of preparing unsaturated dimers of .alpha.-methylstyrenes which are suitably used as agents for controlling molecular weight in the production of copolymers, such as SBR.
2. Description of Related Art
In production of copolymers, such as acrylonitrile butodiene-styrene (ABS) resins, acrylonitrile styrene (AS) resins, polystyrene and styrene butadiene rubber (SBR), tetrachloromethane or dodecylmercaptans have been used as agents for controlling molecular weight. However, as to tetrachloromethane, which destroys the ozone layer, its abolition by the end of 1995 was settled at the Montreal International Conference in 1992, and the use of dodecylmercaptans has come to be restricted due to regulations concerning odor pollution caused during handling and uses. These agents therefore have come to be substituted by unsaturated dimers of .alpha.-methylstyrenes, particularly by 1-pentene dimers, but there is not yet reported any industrial method for producing 1-pentene dimers in good selectivity.
In Japanese Patent Application Kokoku Koho (publication) No. 41-6335 is disclosed a conventional method of preparation of unsaturated dimers of .alpha.-methylstyrenes wherein .alpha.-methylstyrenes are dimerized by using sulfonic acids as catalysts. The method however is poor in the selectivity toward 1-pentene dimers since the catalysts are in direct contact with the .alpha.-methylstyrenes. Further, since the resulting unsaturated dimers of .alpha.-methylstyrenes decompose readily on distillation with heat under acidic conditions caused by the presence of the catalysts, the method needs complicated steps preceding to the distillation, for example, removal of the catalysts from reaction solutions by filtration, by neutralization with a large amount of an alkali or by rinsing with water or an aqueous alkali solution. Even the unsaturated dimers resulting from such complicated steps and distillation involve a problem. That is, the unsaturated dimers purified by the distillation are free of problems for a while, but are so poor in preservative property as to get colored during storage.